Here in Seattle, breweries fill Corny's. You have to drop them off and pick them up full the next day. They'll even wash them for you.
All states are different... if it's legal to fill corny's in your state, then obviously that's the easiest thing to do. Don't go out and buy a Sanke conversion, pay the deposit and buy the beer in a 1/6 bbl keg. That would be silly. Like Bmick said, call and ask.